1. Field of the Invention
The invention relates to a phosphor and a light emitting device comprising the same, and particularly to a phosphor and a light emitting device having high luminous brightness.
2. Description of the Related Art
In recent years, light emitting devices utilizing semiconductors for emitting lights have been intensively used. In particular, light emitting diodes (LED) have been developed successfully. Compared to conventional light emitting equipments, such as a cold cathode fluorescent lamp and an incandescent lamp, the light emitting devices utilizing light emitting diodes have advantages of higher emitting efficiency, smaller volumes, lower power consumption, and lower cost. Therefore, such light emitting devices are used as various light sources. A semiconductor light emitting device comprises a semiconductor light emitting element and a phosphor. The phosphor can absorb and convert a light emitted from the semiconductor light emitting element. The light emitted directly from the semiconductor light emitting element and the light converted and emitted from the phosphor can be mixed for use. The light emitting devices can be used in various applications, such as fluorescent light, car lighting, display devices, and backlight for liquid crystal displays. White light emitting devices are used extensively. Currently, a white light emitting device is formed by using YAG:Ce phosphor (Y3Al5O12:Ce), the element Ce being the active center, together with a semiconductor light emitting element for emitting a blue light. However, the chromaticity coordinates of the mixed lights from the YAG:Ce phosphor and the blue-light semiconductor light emitting element is on a junction line between the chromaticity coordinates of the blue-light semiconductor light emitting element and the YAG:Ce phosphor. Therefore, the emitted mixed light is a white light lacking a red light, and of which the color rendering property and the color saturation property are apparently insufficient. In addition, the YAG:Ce phosphor has a major excitation spectrum area inconsistent with the light emitting region of the semiconductor light emitting element, thus conversion efficiency of the excited light is poor, and obtaining a white light source with high luminous brightness is difficult. For solving the issues of poor color hue and low luminous brightness, the YAG:Ce phosphor mixed with a phosphor for emitting a red light has been developed, and the quality of the red light phosphor has been improved, thereby increasing luminous brightness.
However, phosphors absorbing a blue light and converting it into a red light are rare. The developments and researches of such kind phosphors are focused on nitride and oxynitride phosphors. Currently known phosphors include Sr2Si5N8:Eu phosphor using the element Eu as the active center, CaAlSiN3:Eu phosphor, and sialon phosphor having a formula of MzSi12−(m+n)Alm+nOnN16-n:Eu. However, luminous brightness of the Sr2Si5N8:Eu phosphor is poor, and thus applications of the Sr2Si5N8:Eu phosphor are limited and are not widely used. The sialon phosphor has adequate durability, but of which the luminous brightness is poor, thus the sialon phosphor is not popular. The CaAlSiN3:Eu phosphor has better durability and better brightness compared to the sialon phosphor; however, there is still a need for further increasing the luminous brightness of phosphors for providing light emitting devices with higher light emitting efficiency.